This invention relates to the structure employed in the wellhead portion of a subsea drilling system and, more particularly, to a seal assembly operable for use in a conventional wellhead/casing hanger system to effect a sealing of the annular opening formed between the wellhead housing and a casing hanger by virtue of the spacing that exists between the inner wall surface of the former and the outer wall surface of the latter.
In accord with the teachings of the prior art, it is known to construct a subsea drilling system by means of the interconnection of a plurality of mutually cooperative subsystems. Moreover, by way of illustration in this regard, it has been known heretofore to effect the construction of a subsea drilling system by interconnecting together three subsystems; namely, a marine riser system, a well control system and a wellhead system. In accord with the nature of this form of construction, the three named subsystems are suitably interconnected such that the marine riser system extends from essentially the surface of the sea to a point therebelow, the wellhead system is suitably positioned on the sea bottom, and the well control system is interposed in interconnected relation between the marine riser system and the wellhead system.
In a word, the function of each of the three above-named subsystems in a subsea drilling system is in substance as follows. The marine riser system is basically intended to perform three functions. Namely, it serves as a conduit for the return of drilling fluids, it operates as a guide for drilling tools and casing strings, and it is employed to run and install the blowout preventer stack. The well control system, otherwise known as the blowout preventer stack, provides the means to control sudden changes in well bore pressures during drilling operations. Finally, the wellhead system functions as a pressure vessel to which casing strings are securely sealed in the course of the performance of the drilling operation.
In further regard to the wellhead system, the latter is sometimes also referred to as a wellhead/casing hanger system. Typically, such a system is comprised of at least a wellhead housing and a casing hanger, with the latter designed to be positioned within the former. When the casing hanger is so positioned relative to the wellhead housing, an annular opening is formed therebetween. More specifically, this annular opening represents the spacing, i.e., the clearance, that necessarily exists between the inner wall surface of the wellhead housing and the outer wall surface of the casing hanger. The desirability of packing off this annular opening, i.e., effecting a sealing thereof, as well as the reasons therefor are well known to those skilled in this art, and, accordingly, it is not deemed necessary to iterate them herein.
The matter of effecting the packing off of the aforereferenced annular opening that is present between the wellhead housing and the casing hanger has been addressed in the prior art. More specifically, as taught, for example, in U.S. Pat. No. 3,797,864--Hynes, et al., which is assigned to the same assignee as the present invention, it is known to interpose an elastomeric seal device between the wellhead housing and the casing hanger to effect a packing off, i.e., a sealing of the clearance existing therebetween. In accord with the teachings of this patent, the subject elastomeric seal device includes an upper metal seal having downwardly extending lips projecting therefrom, a lower metal seal having upwardly extending lips projecting therefrom, and an elastomeric seal ring supported between the aforementioned upper and lower metal seals. The mode of operation of this elastomeric seal device is such that in response to a force being applied thereto metal-to-metal contact is established between the lips of the metal seals and the corresponding wall surfaces of the wellhead housing and the casing hanger. In addition, a deformation of the elastomeric seal ring is effected such that the latter is also forced into engagement with the wall surfaces of both the wellhead housing and the casing hanger.
Although from an operational standpoint, the elastomeric seal device constructed in accordance with the teachings of the Hynes, et al. patent has performed satisfactorily, improvements in the construction thereof are still to be desired. Namely, it has been found that, in order to energize the seal with the aforedescribed elastomeric seal device to pack-off the annular clearance between the wellhead housing located underwater and the respective casing hangers located in this housing, commonly has required the application to the seal device of a torque force on the order of 20,000 ft.-lbs. More specifically, it has taken the application of a torque force of this magnitude in order to effect the seal desired, i.e., a 10,000 psi seal, when an elastomeric seal device constructed in accordance with the teachings of the Hynes, et al. patent is employed.
Inasmuch as the elastomeric seal device, which has been described above, is commonly employed at a substantial distance below the surface of a body of water, it is necessary that means be provided for transmitting a torque force of the requisite magnitude through a considerable distance before it can be applied to accomplish the energization of the subject seal device. Accordingly, it would be viewed as a favorable attribute of any seal device employable in the wellhead portion of a subsea drilling system, if it were possible, through the use thereof, to realize a measurable reduction in the magnitude of the torque force required to effect a seal of a given pressure from that needed when utilizing other forms of the same functional type of seal device to achieve a seal of the same pressure. Moreover, the ability to accomplish the energization of a seal device of the subject type with a reduced amount of torque force becomes even more important as the distances at which it is desired to conduct subsea drilling operations continue to increase. Namely, the feasibility of transmitting a torque force of a given magnitude decreases as the distance, through which the torque force is required to be transmitted before the application thereof, increases. The reasons for this are many and varied, but are well known to those skilled in the art of subsea drilling systems. Accordingly, it is not deemed necessary to state them at this point.
Once having been emplaced in sealing relation relative to a wellhead housing and a casing hanger, it is normally intended that the elastomeric seal device constructed as described above will remain so positioned. However, on occasion, the need arises to effect the removal of this elastomeric seal device from between the wellhead housing and the casing hanger. The elastomeric seal device, which forms the subject matter of the afore-referenced Hynes, et al. patent, has, unfortunately, proven to be disadvantageously characterized insofar as concerns effecting the removal thereof from the wellhead portion of a subsea drilling system. Namely, once having been energized, as referred to previously hereinabove, such that metal-to-metal contact is established between the lips of the metal seals and the wall surfaces of the wellhead housing and the casing hanger, and such that the elastomeric seal ring has undergone deformation so as to be in engagement with the same wall surfaces of the wellhead housing and the casing hanger, difficulties have been encountered in some instances in effecting the removal of the elastomeric seal ring from between the wellhead housing and the casing hanger. More specifically, the nature of the difficulty, which has been encountered in this regard, resides in the fact that the elastomeric seal ring has evidenced a susceptibility to sever when force is applied to the upper metal seal in an effort to remove the elastomeric seal device from its sealing position. That is, a severing of the elastomeric seal ring has occurred whereby the lower metal seal remains emplaced in the position which the latter has occupied for purposes of effecting the packing off of the clearance opening between the wellhead housing and the casing hanger. A need has thus been evidenced in the prior art for a seal device that would be capable of being removed from between a wellhead housing and a casing hanger when the need for such removal arises, without concomitantly being disadvantageously characterized, as is the Hynes, et al. elastomeric seal device, by the tendency of the elastomeric seal ring to sever leaving the lower metal seal still emplaced.
In summary, there has been shown to exist in the prior art a need for a new and improved seal device capable of being employed in a subsea drilling system and operable therein for purposes of effecting the packing off of the annular clearance between the underwater wellhead housing and the respective casing hangers, which are landed in the housing. Moreover, a need has been shown for such a device, which would be characterized by the fact that it is capable of being employed in the manner of the Hynes, et al. elastomeric seal device previously referred to hereinabove, while at the same time being advantageously characterized relative thereto in the fact that the new and improved seal device is capable of being energized with a torque force of lesser magnitude than the Hynes, et al. elastomeric seal device while retaining the capability of exerting a seal of the same pressure as the Hynes, et al. elastomeric seal device, as well as in the fact that, should the need arise to remove the new and improved seal ring from its sealing position relative to the wellhead housing and the casing hanger, it can be so removed without difficulty, i.e., without fear that portions thereof will be left emplaced between the wellhead housing and the casing hanger.
It is, therefore, an object of the present invention to provide a new and improved form of seal assembly operable for effecting a seal between a pair of spaced surfaces.
It is another object of the present invention to provide such a seal assembly, which is particularly suitable for use in the wellhead portion of a subsea drilling system.
It is still another object of the present invention to provide such a seal assembly that is operable to effect the packing off of the annular clearance between an underwater wellhead housing and the respective casing hangers, which are landed in the housing.
A further object of the present invention is to provide such a seal assembly which is characterized in the fact that it is capable of providing maximum seal integrity over extended periods.
A still further object of the present invention is to provide such a seal assembly which is energizable in response to the application thereto of an energizing force of a significantly lesser magnitude than that required heretofore by prior art forms of seal assemblies that have been employed for similar purposes.
Yet another object of the present invention is to provide such a seal assembly which embodies first and second means that are cooperatively associated one with another so as to be operative to establish a lost motion connection therebetween when the subject seal device is in a state of enegization, and so as to be operable to form a rigid connection therebetween when a need arises to effect the removal of the subject seal device from between a wellhead housing and a casing hanger.
Yet still another object of the present invention is to provice such a seal assembly which is relatively inexpensive to provide and easy to employ, while yet being compatible for use in subsea drilling systems of known construction.